As is known, one of the needs which has always been mostly felt by the producers of articles made of plastics material, and in particular by producers of lenses for glasses, is the need of marking their products, both to distinguish each article within their own production lines and to provide clients with "customized" products with a logo or mark chosen by the clients themselves.
In order to satisfy this need, several techniques have been proposed in the art, all essentially based on making marks printed or engraved on the external surface of the substrate.
However, these techniques have not proved entirely satisfactory, as the logo or pattern marked on the lens and visible to the naked eye, may not only result to be aesthetically poorly attractive, but can also in some cases alterate the optical performances of the lens in the zone nearby the mark.
Besides, the printing or engraving techniques are often troublesome and difficult to control, with a remarkable increase in production costs.
In order to try to obviate somehow these drawbacks, the use of fluorescent dyes to form marks visible only when the substrate is lit with ultraviolet rays has been proposed, as is described, for example in Japanese Patent JP 60023092 or U.S. Pat. No. 4,257,692.
Although these techniques have allowed to make a logo or mark not visible under the normal use conditions of the articles so marked, improving both the aesthetical characteristics and the optical quality of the latter, their realization is anyhow delicate and difficult to control.
Besides, the marks so obtained may be visualized only by means of special lighting devices that are not usually available to the public.
As an alternative solution, it has been proposed to form marks that may be visualized by moisture condensation on the substrate, for instance by breathing thereon, as is described, for example, in European patent EP 0 031 633.
In this case, the mark is formed by causing a zone having a prefixed pattern of the substrate--precisely a zone intended to constitute the mark--to be more hydrophilic that the surrounding ones, so that by breathing thereon the surface of the substrate is fogged except for a pre-defined portion forming the desired mark.
According to such patent, the wanted improved characteristics of water affinity (hydrophily) of the zones forming the mark are achieved by applying to the same a concentrated solution of sulfuric acid.
Even though this technique allows to make marks identifiable by simply breathing over, the same is however substantially limited to substrates without any colouring either on the surface or in mass.
In fact, it has been found that sulfuric acid irreversibly alterates substantially all the traditional organic dyes used for colouring the plastics materials used in the optical sector.
Besides, the application of concentrated solutions of sulfuric acid--notoriously corrosive and to be cautiously handled--is not easy and not without risks for the operator.